Silicon wafers produced by processing a nitrogen-doped silicon single crystal produced by the Czochralski method (CZ method) into wafers and then subjecting the wafers to heat treatment (it may be called annealing) at a high temperature for a long period of time in an inert gas atmosphere such as argon are recently attract attentions as products of which gettering ability for metal impurities and so forth is enhanced by increasing integrity of device active layer at wafer surface to make a region of at least about 3 μm from the surface defect-free, which is required for the device fabrication, and increasing density of oxide precipitates in the bulk, called BMD (Bulk Micro Defect) and serving as gettering sites.
Although the current mainstream of products applied with this technique is constituted by wafers having a diameter of 200 mm or less, it is attempted to apply this technique to wafers having a diameter of 300 mm currently under development, and it is estimated that demands for wafers having a diameter of 300 mm or more will be increasingly expanded in future.
However, cooling of a pulled crystal having a diameter of 300 mm or more does not advance easily, because a pulling apparatus used for the production thereof has a quite large volume and thus a large heat capacity and because of influence of specific heat. It has been revealed that, because of the above, the size of void defects (also called COP) in the silicon single crystal becomes larger, and the void defects can be eliminated only from a surface layer of an extremely small thickness with the standard conditions for the subsequent annealing (1200° C., 1 hour, argon atmosphere) even for a nitrogen-doped single crystal wafer.
Although the phenomenon that cooling of a crystal does not advance easily was similarly observed upon the alternations of generations in the past for wafer diameter (for example, upon the turning point of diameter from 150 mm to 200 mm), the influences thereof were not so significant compared with those of marked increases of volume of pulling apparatus and crystal volume at the time of the expansion of the diameter from 200 mm to 300 mm, and as a result, the void defects could be eliminated for a sufficient depth (at least 3 μm) with the conventional standard annealing conditions (1200° C., 1 hour, argon atmosphere).
Further, the so-called grown-in defects introduced upon pulling of crystals have been actively studied from only several years ago for the first time. At that time, wafers having a diameter of 300 mm were not produced at a mass production level at all and production of a sample of that shape was just became possible during that period. Therefore, it was not conceived to eliminate grown-in defects by annealing of wafers having a diameter of 300 mm, and there has hitherto been much less expected the phenomenon that the elimination of grown-in defects by annealing of wafers having a diameter of 300 mm becomes much more difficult compared with that for wafers having a diameter of 200 mm.